This invention relates to a method and composition for preventing for an extended period of time fouling of submersed or submerged objects or marine structures while also minimizing pollution and more particularly to a method and composition consisting of polyurethanes and similar castable polymers necessary for target strength reduction. The incorporation of organotin compounds into polyurethanes has heretofore been unsuccessful.
From the beginning of man's attempt to use water to travel, he has been plagued with the problem of his ships, buoys, pilings, or other objects placed in the water, being fouled by organisms present in the water. It has been found that microorganisms, their viscous bio-organic products and absorbed organic matter, constitute a tenacious, opaque slime which forms on these submerged surfaces. The initial organisms in this fouling sequence are bacteria followed by a biotic progression of diatoms, hydroids, algae, bryozoans, protozoans, and finally macrofoulants, such as barnacles.
The resultant effect of a concentration of these plants and animals settling and attaching themselves to ships are well known: they contribute significantly to speed reduction; they increase fuel consumption, and, in the area of concern of water craft detection they strengthen the noise signature of vessels under way thereby rendering covert activity more difficult. Also, fouling of sonar domes has been found to seriously limit the active and passive modes of operation of ships' acoustical systems.
This problem of marine growth (fouling) applies not only to vessels but also to other submersed and submerged objects. For example, fouling of moored data systems and ship-and-shore facilities by marine organisms impedes operations and necessitates a large maintenance allocation. Buoys shift due to the excessive weight of fouling organisms. Effective operation of sonars is hampered by the self-noise generated by the fouling of sonar dome surfaces. Wood pilings in berthing facilities undergo structural weakening and ultimate destruction due to marine borer and fungal attack.
It is only since the beginning of this century that improvements have been made in the early Phoenician methods of using copper cladding and poisonous paints to prevent fouling. Some of these improvements include the sue of asphalt as an antifouling coating and paints containing copper, or salts, or organic derivatives of tin, lead, mercury, arsenic or phosphorus, as well as organic toxic materials such as salicylanilides, chlorinated hydrocarbons, polychlorophenols, and aryl or alkyl nitriles. Of these, mercury, arsenic, and lead are no longer used because of the occupational hazards associated with their manufacture and application, their toxicity to humans and non-target marine life and because tin compounds are far better. Copper compounds have the disadvantages of promoting galvanic corrosion, especially of aluminum hulls, and also they do not repel algae along the waterline. Most if not all paints have the disadvantages of a fast and uncontrolled leaching rate which depends on external factors such as coating age, ship velocity, salinity, temperature and the composition of the primary slime layer. The fast leaching rate, in turn, results in concentrations of the toxicant well above normal oceanic background and well above what is needed to prevent fouling (so called "overkill") and excessive impact on the environment. It also results in fast depletion of the toxicant and shortens the service life of the paints which necessitates frequent repainting. In addition, some paints lack durability and integrity. The need to peel or blast off old paint before repainting results in further pollution of the environment. The total cost of fouling runs in many hundreds of millions of dollars and solutions better than paints are urgently needed.
Such improvement came about with the inventions describing chemically bonding organotin compounds to polymeric materials, either by attaching them to polymeric backbones via ester groups or by incorporating them into elastomers whether or whether not such incorporation is by chemical bonding by a covulcanization process. The end products are either structural materials such as glass-reinforced or non-reinforced plastics, and rubbers. Such structural materials do not need painting by antifouling paints because they already contain the organotin moiety bound to a polymeric backbone and capable of breaking off by hydrolysis and poisoning the offensive organisms. The improvement consists of a low leaching rate resulting in avoidance of excessive toxicant emission, avoidance of "overkill", reduction of the environmental impact by a factor of 10 more, and extension of the service life of such paints, or structural materials not needing painting, to a service life of 5 to 10 from the typical 11/2 or 2 years available from older type paints. The reasons for this improvement are believed to be due to the leaching rate being governed by the rate of hydrolysis of the ester group linking the organotin moiety to the polymer backbone, said rate being independent, or less dependent, on external factors such as salinity, speed, primary slime layer and other factors than the mechanism of leaching and defoliation characteristics of conventional paints. Another theory assumes that hydrolysis is catalyzed by the body fluids of the attaching fouling animal. Materials for which this latter theory is true would have ideal antifouling properties consisting of a surface toxic only to the attaching fouling organism but almost completely non-polluting to the environment. In either case the carboxylic groups on the polymer chain which are generated through hydrolysis make the polymer hydrophilic enough to dissolve and regenerate a new toxic surface. Regardless of theoretical consideration, field tests have shown the new generations of antifouling materials far better than conventional paints in terms of reduced environmental impact and increased service life.
Examples of new structural antifouling material not needing painting are vinyl polymers such as acrylates and methacrylates, polyesters, epoxies, alkyds, and maleic anhydride copolymers described among others in U.S. Pat. No. 4,082,709 and elsewhere, some of said polymers being suitable for glass reinforcement of for transparent film formation. Other elastomeric antifouling materials such as natural rubber, neoprene (polychloroprene rubber), butyl rubber (isobutyelene/isoprene copolymer rubber), SBR (styrene/butadiene rubbers), polybutadiene rubbers such as cis-polybutadiene rubber, synthetic polyisoprene rubbers such as cis-polyisoprene or synthetic natural rubber, nitrile rubbers (butadiene/acrylonitrile copolumers), ethylene/propylene/dicyclopentadiene and other ethylene/propylene/diene terpolyemrs, and others are described in U.S. Pat. No. 3,639,583.
It has however so far not been possible to synthesize structural antifouling polyurethanes and similar materials although paints in which the polymeric material is a solvent-soluble polyurethane have been described below as exemplified by U.S. Pat. No. 4,554,185. The difficulties to synthesize such structural antifouling polyurethanes are surmised to be due to the catalytic effects that some organotin compounds have on the reaction of isocyanates and thioisocyanates with polyols and other suitable reactants. This causes a run away polymerization which is manifested by an exotherm and premature gelation which reduces the pot life to unmanageable short durations. Such difficulties are not be expected in paints as described in U.S. Pat. No. 4,554,185 in which the inventors have kept the isocyanate concentration low (never above 5.24%). Consequently the occurrence of a runaway polymerization is less likely even if organotin compounds having a catalytic effect on the polyurethane reaction are present. In addition if runaway polymerization does occur it would not render the paints less useful since in a paint, in contrast to a structural material, there is no need for a pot life. The inventors of U.S. Pat. No. 4,554,185 specify that either the isocyanate or the diol must be monofunctional in order to prevent crosslinking. This avoidance of crosslinking is needed so that polymeric material of the paint shall remain soluble in its solvent. This a paint is applicable to the surfaces to be painted regardless of whether or not runaway polymerization occurs. In contrast to this a thermosetting polymer, such as the one of our invention, which is intended to set to a structural material ought to be crosslinked. This requires that both monomeric reactants be multifunctional and have a pot life which is long enough to enable mixing and pouring into a mold. This in turn requires absence of impurities which will cause runaway polymerization.
Structural polyurethanes have the advantages of castability, sprayability, excellent physical properties and great versatility enabling varying the consistency of the final product from a soft rubber to a hard plastic or a flexible or a rigid foam. They have recently been considered as candidate materials for target strength reduction. Thus, there is a continuing need for further development utilizing the incorporation of proper toxic materials into polyurethanes. This invention accomplishes these goals.